In situ conservation measures—including the
creation and management of protected areas,
increasing connectivity between wildlife populations, and reduction of the impacts of predation and hunting—can achieve some success
where the amount of habitat remaining is sufficient for viable populations (6). Increasingly,
however, more intensive forms of threatened species management are required to address local
extinctions and impending threats to critical areas
of habitat. Progress in reversing defaunation is
emerging from conservation translocations—the
intentional movement of animals to restore populations (7) (Fig. 1).

Population restoration: Reintroduction
and reinforcement

The intentional movement and release of animals has occurred for millennia, but the use of
translocations to address conservation objectives is barely 100 years old (8). In recent decades,
there has been an increase in the number of
species that are the focus of conservation translocations to restore and enhance populations;
for vertebrates alone, at least 124 species were
translocated during 1900–1992, and this had risen
to 199 species by 1998 and to 424 species by 2005
(9). Two types of translocation for population
restoration are recognized: (i) reinforcements,
involving the release of an organism into an existing population of conspecifics to enhance population viability, and (ii) reintroductions, where
the intent is to reestablish a population in an
area after local extinction (7) (Fig. 1). The critical
feature of these translocations is the release of
animals into their indigenous range, the known
or inferred distribution derived from historical
records or other evidence (7).

Previous work has shown that conservation
translocation projects, as with other types of
conservation management, show a marked taxonomic bias toward birds (33% of projects, whereas
birds make up 18% of species represented in
nature) and mammals (41% of projects versus
8% of species), particularly the larger, more charismatic species, almost irrespective of the degree
of threat or vulnerability (10). Recent data on
reinforcements show that this bias toward birds
and mammals is continuing (11). For conservation translocations in general, relatively few
invertebrate, reptile, amphibian, or fish species
are represented relative to their prevalence in
nature (Fig. 2). The global distribution of species’
translocations suggests a geographic bias also,
with most activity in developed regions (Fig. 2).

The ultimate objective of any reintroductionis the establishment of a self-sustaining popu-lation and, using this definition, reviews of re-introduction outcomes have indicated generallylow success rates (12), as low as 23% (13). Con-cern over high failure rates prompted analysesof the factors associated with translocation suc-cess. In 1989, the first comprehensive reviewlooked at the reintroduction and reinforcementof 93 species of native birds and mammals (12).This data set was updated, and 181 mammal andbird programs were reanalyzed in 1998 (14). Bothstudies identified habitat quality at the releasesite, release into the core of a species range, andtotal numbers released as determinants of suc-cess (12, 14). An independent analysis of a broadertaxonomic range of animal translocations over20 years highlighted the greater likelihood ofsuccess associated with the release of wild versuscaptive animals and confirmed the importanceof larger founder group sizes (13).Reintroduced populations go through a pe-riod of relatively small population size where therisks of inbreeding and loss of genetic variationis high; the challenge, therefore, is to minimizeloss of genetic variation by creating large effec-tive population sizes (15). The key determinantsof the genetic diversity retained in a reintroduc-tion will be the total number of founders andthe proportion contributing genetically to thenext generations (16). Thus, even when a largepopulation results, there might be considerableloss of genetic diversity during the early stagesof population establishment (17), and the num-ber of founders necessary for preservation ofgenetic diversity might be substantially greaterthan that required for population establishmentand growth (18). Low initial population sizes mightalso make reintroduced populations vulnerableto Allee effects, which might have contributed topast reintroduction failures, although this linkhas not been shown (19). Reinforcement of exist-ing populations can increase population size,prevent Allee effects, and increase genetic diversity,but also carries a risk of loss of local adaptationand the introduction of pathogens, particularlyfrom captive breeding programs (11).

Simple classification of any reintroduction
as success or failure to result in a self-sustaining
population is of limited use because the time
scale for success evaluation is important, and
there are examples of successful projects failing
at a later stage (13). The International Union
for Conservation of Nature (IUCN) guidelines
advocate that projects make clear definitions
of success in relation to three phases of any reintroduction: establishment, growth, and regulation, with future population persistence assessed
through population viability analysis (7). Assessment of success or of the causes of failure can be
made only through adequate postrelease monitoring (20). Monitoring is needed also to facilitate meta-analyses (13), to track genetic diversity
(16), and to evaluate the performance of reintroduced populations and the possible impacts on
recipient ecosystems (21).

Conservation introductions

Perhaps the greatest challenge facing practi-tioners of species or ecosystem restoration isthe definition of a target state (22). Attempts toreturn a system to some historical conditionmake somewhat arbitrary decisions about howfar back in time to go. Historical restorationreference states vary according to the historyof human occupation, with pre-European set-tlement conditions often held up as the base-line (23). However, a desire to return to somepast state makes some assumptions, includ-ing the implication that near-pristine condi-tions existed in pre-European times and thathistorical restoration targets will be sustain-able with changing climate (22). It is now rec-ognized that past species distributions do notindicate current suitability and that currentspecies’ distribution does not guarantee fu-ture suitability (24). Climate change, in tan-dem with human-facilitated species invasionsand land transformation, contribute to thecreation of novel ecosystems: systems that dif-fer in composition and function from pastsystems (25).

If we acknowledge that restoration and maintenance of species within their indigenous ranges
will remain a foundation of conservation efforts, the realization that a return to a completely natural world is not achievable frees us
to think about more radical types of conservation translocation. Conservation introductions
involve the movement and release of an organism outside its indigenous range (7). Two types
of conservation introduction are recognized by
the IUCN: assisted colonization and ecological
replacement (Fig. 1).

Assisted colonization

In 1985, Peters and Darling (26) suggested that
climate change might alter habitat suitability
for species confined within protected areas, effectively stranding them in increasingly unsuitable sites. They proposed the translocation of
individuals into new reserves encompassing habitat that was or would become appropriate. Possibly because of the low profile of global climate
change, the unreliability of early predictive models of climate, and the radical nature of the proposal, the idea of proactive translocation initially
gained little traction (27). However, there is growing acknowledgment that conservation managers could take action to address climate-induced
changes in species’ habitats where individuals of
affected species are unable to naturally colonize
new areas as habitat suitability shifts (28–30).
Understandably, given the devastating ecological
impact wrought by invasive species, assisted colonization has been greeted with extreme skepticism, which has promoted a vigorous debate
in the literature (31, 32). The 2013 IUCN guidelines
define assisted colonization in broad terms as
the intentional movement of an organism outside its indigenous range to avoid extinction of
populations due to current or future threats (7).
Under this definition, far from being a radical
new translocation approach, assisted colonization is already being applied as a conservation
tool in Australasia to protect, on predator-free
islands, populations of species, such as the kakapo